(PECL memcached >= 0.1.0)
Memcached::prependByKey — Prepend data to an existing item on a specific server
$server_key
   , string $key
   , string $value
   )
    Memcached::prependByKey() is functionally equivalent to
    Memcached::prepend(), except that the free-form
   server_key can be used to map the
   key to a specific server.
  
server_key
     The key identifying the server to store the value on or retrieve it from. Instead of hashing on the actual key for the item, we hash on the server key when deciding which memcached server to talk to. This allows related items to be grouped together on a single server for efficiency with multi operations.
key
     The key of the item to prepend the data to.
value
     The string to prepend.
   Returns TRUE on success or FALSE on failure.
   The  Memcached::getResultCode() will return
   Memcached::RES_NOTSTORED if the key does not exist.